1. Field of the Invention
The present invention pertains to linear amplification of RF signals, for example linear amplification of RF signals using a multicarrier amplifier.
2. Related Art and Other Considerations
Amplifiers are typically employed to amplify RF signals in order to provide, e.g., increased power for transmission purposes, particularly transmission over an air interface to a receiver such as (for example) a mobile station (e.g., a user equipment unit (UE) such as a cell phone). But in amplifying an input RF signal, the amplifier may add unwanted components due to non-linear characteristics of the amplifier. Such is particularly true when the type of amplifier utilized is chosen for its power efficiency and/or when plural continuous wave RF input signals are applied to the amplifier. Rather than just producing amplified signals corresponding input signals, such amplifier may also output certain additional signals related to the frequencies of the input signals. In this regard, mathematically the output of the amplifier can be expressed as a DC term; a fundamental term (which includes nominal gain for the input signals and an amplitude distortion); and (typically second and third) harmonics terms. The DC term and harmonics can usually be filtered out rather easily, leaving a passband.
The distortion within the passband is not easily removed, but rather is minimized by designing the overall amplifier system in order to compensate for the non-linear characteristics of the amplifier component per se. Such xe2x80x9clinearizationxe2x80x9d of an amplifier system is important in order to avoid distorted signal trajectories and to avoid errors in determining the logic level of individual digital signals.
There are many techniques that can be used to linearize amplifiers. Among the linearization techniques are the following: Back off (in the case of Class A amplifiers); Feedforward; Vector summation; Predistortion, and Feedback. Several of these linearization techniques are briefly described in U.S. Pat. No. 6,075,411 to Briffa et al., which is incorporated herein by reference in its entirety. See also, in this regard, Briffa, Mark, xe2x80x9cLinearisation of RF Power Amplifiers,xe2x80x9d 1996.
The feedforward technique is advantageous for broadband linear RF amplifier systems. As mentioned briefly above, since the multicarrier input signal is distorted by the non-linearities in the main amplifier, certain intermodulation (IM) products appear at the output. In essence, the feedforward technique generates an error signal by comparing the input signal with the main amplifier output. The error signal is subtracted from the main amplifier output, leaving a (nearly) distortion-free amplified signal.
FIG. 1 illustrates a simplified, example amplifier system 20 which employs a feedforward technique to minimize distortion. The amplifier system 20 comprises a phase and gain adjuster 22 which receives, via coupler 24, an input signal. Output from the phase and gain adjuster 22 is applied to main power amplifier 26. Output from main power amplifier 26 is applied to a coupler 28, and from one leg of coupler 28 via attenuator 30 to subtractor 32. Both subtractor 32 and first loop controller 34 receive, via delay 36, the input signal as obtained from coupler 24. Output from subtractor 32 is applied both to first loop controller 34 and to a second gain and phase adjuster 40. Output from gain and phase adjuster 40 is applied to auxiliary amplifier 42, whose amplified output is coupled by coupler 44 to line 46. Line 46 emanates from coupler 28 and delay 48. The output signal carried on line 46 at point 51 is applied via coupler 50 and attenuator 52 to third loop controller 54, with third loop controller 54 connected to control gain and phase adjuster 40.
Being in a simplified form for sake of illustration, the amplifier system 20 of FIG. 1 comprises three loops. A first loop of amplifier system 20 includes phase and gain adjuster 22, main power amplifier 26, coupler 28, attenuator 30, and subtractor 32. If the gain and phase shift through phase and gain adjuster 22, main power amplifier 26, and attenuator 30 equals the gain and phase shift through delay 36, an error signal indicative of the distortion of main power amplifier 26 is output by subtractor 32. But in order to equalize gain and phase shift through these paths, first loop controller 34 is used to produce control signals, applied on line 60, to phase and gain adjuster 22.
A second loop of amplifier system 20 comprises attenuator 30, subtractor 32, gain and phase adjuster 40, auxiliary amplifier 42, coupler 44, and delay 48. If the gain and phase shift through attenuator 30, subtractor 32, gain and phase adjuster 40, and auxiliary amplifier 42 equals the gain and phase shift through delay 48, except for a 180 degree phase shift, the distortion is added in opposite phase at coupler 44, thus canceling out the distortion of main power amplifier 26 on line 46. A third loop including attenuator 52 and third loop controller ensures phase and gain equality in these two paths.
Thus, the first loop described above with reference to amplifier system 20 creates an error signal which contains the intermodulation distortion from the main power amplifier 26. The second loop serves to cancel intermodulation distortion at output point 51, while leaving the carriers unaffected.
The characteristics of the components, especially of the power amplifier, can vary considerably due to manufacturing tolerances, temperature changes, and aging. Gain variations of several decibels (dB) are not uncommon. A system as described above can, in principle, accommodate for any gain change, with the phase and gain adjuster compensating for the gain variation. This means, however, that the full dynamic range of the phase and gain adjuster cannot be utilized.
In the above regard, a four-quadrant quadrature phase and gain adjuster has, in general, a relatively high output noise level with the noise being more or less independent of the gain setting. There are exceptions, such as phase and gain adjusters comprising Gallium Arsenide or PIN-diode components. However, the Gallium Arsenide-employing phase and gain adjusters are considerably more expensive than a noiser phase and gain adjuster fabricated with silicon technology. A PIN-diode type phase and gain adjuster may be too slow in its control function for many applications.
In general, a silicon-based phase and gain adjuster typically has 6-10 dB higher output noise than a variable gain amplifier for the same current drain. As an example illustration of this general proposition, assume that a silicon-base phase and gain adjuster in a control range of 6 dB has an output signal to noise ratio of 150 dB at maximum gain. When the gain is 6 dB down, the signal to noise ratio falls to 144 dB. On the other hand, a variable gain amplifier operating in the 6 dB control range would likely have an output signal to noise ratio of 158 dB at maximum gain, and a signal to noise ratio of 152 dB when the gain is 6 dB down.
Undesirable noise attending amplifier performance is significant in various applications, such as (for example) wireless telephony. In this regard, specifications for mobile phone systems typically restrict the amount of noise produced by a transmitter outside the transmitting channel, especially in the receiving band. If the undesired noise can be sufficiently limited, costly and space consuming noise rejection filters can be avoided.
Since the phase and gain adjuster is critical for dynamic range, some other form of gain control has to be employed in order to limit noise. This other form of gain control can be, for example, manufacturing adjustments. Measuring the temperature characteristics and insertion of some element with opposite temperature characteristics can compensate for the variations caused by temperature changes. In some cases, every individual system has to be characterized and calibrated, adding cost to the overall system.
What is needed, therefore, and an object of the present invention, is a simple technique for minimizing or lowering output noise of an amplifier system for radio communications.
An amplifier system for radio frequency signals comprises a combination of a phase and gain adjuster and a first amplifier. The combination receives an input signal and generates a first amplified signal having a gain adjusted in accordance with a gain of the phase and gain adjuster. The first amplifier is preferably a variable gain amplifier. A main amplifier (e.g., second amplifier) receives the first amplified signal and generates a second amplified signal. A first controller uses a signal derived from the second amplified signal to generate a first control signal. The first control signal is applied to the phase and gain adjuster to control the gain of the phase and gain adjuster. A second controller is connected to receive the first control signal and to generate a second control signal which is applied to the first amplifier. The second control signal is generated by the second controller ultimately to control the first control signal and thereby control noise of the amplifier system.
With the second control signal being used to control the first control signal, the phase and gain adjuster (which receives the first control signal) can adjust the adjusted signal to make adjustments for a first type of gain variation of the amplifier system. The second control signal is used to enable the first amplifier to change its first amplifier signal to make adjustments for a second type of gain variation of the amplifier system. For example, the first type of gain variation handled by the phase and gain adjuster can be a fast gain variation, and the second type of gain variation handled by the second amplifier can be a slow gain variation. The second type of gain variation can be, for example, a gain variation attributable to one of amplifier system production differences, aging, and temperature variations.
In one aspect, the second controller generates the second control signal so that the first control signal is maintained essentially constant. In one example mode, the second controller generates the second control signal so that a magnitude of the first control signal is maintained essentially constant. In another example mode, the second controller generates the second control signal so that a sum of absolute values of components the first control signal is maintained essentially constant. In yet another example mode, the second controller generates the second control signal so that an absolute value of a large component of the first control signal is maintained essentially constant.
The amplifier system includes further aspects, such as means for producing an error signal indicative of distortion of the main amplifier, and a distortion compensation circuit which uses the error signal essentially to cancel the distortion of the main amplifier in the second amplified signal (e.g., the output of the main amplifier).